Optical sensors are well known in the art, and have utility in a number of different measurement applications. For example, and as shown in FIG. 1, a fiber Bragg grating 10 (FBG 10) formed in an optical fiber 12 or other optical waveguide can be used to measure pressure or temperature. A FBG, as is known, is a periodic or aperiodic variation in the effective refractive index of a core of an optical waveguide, similar to that described in U.S. Pat. Nos. 4,725,110 and 4,807,950 entitled “Method For Impressing Gratings Within Fiber Optics,” to Glenn et al. and U.S. Pat. No. 5,388,173, entitled “Method And Apparatus For Forming Aperiodic Gratings In Optical Fibers,” to Glenn, which are incorporated by reference in their entireties.
FBG 10, when interrogated by broadband light from an optical source/detector 14, will reflect a narrow band of this light (essentially a single wavelength), called the Bragg reflection wavelength, λB, in accordance with the equation λB∝2neffΛ, where neff denotes the index of refraction of the core of the waveguide, and A denotes the spacing of the variations in the refractive index of the core (i.e., the grating spacing). Because strain along the axis of an FBG affects its grating spacing Λ, and because temperature effects both the index of refraction neff and the grating spacing Λ (in the latter case, due to thermal expansion or contraction), FBG 10 can be used as either as pressure or temperature sensor by assessing the magnitude of the shift in its Bragg reflection wavelength. FBG 10 is usually partially transmissive so that a portion of the light at the Bragg reflection wavelength (and light of all other wavelengths that is not affected by the FBG 10) transmits through the FBG 10, which allows further sensors along the optical fiber 12 (not shown) to be interrogated in a multiplexing approach to determine the pressures and/or temperatures present in those locations.
When interrogating the FBG 10, the optical source/detector 14 can be operated in a continuous wave mode, where light is continuously fed to the FBG 10 and its reflections are continuously monitored, or the light can be pulsed. In a pulsed scheme, the frequency of the pulses needs to be sufficiently short to detect changes in the parameter being measured. For example, when measuring temperature in a given application, such as within an oil/gas well, it is noted that temperature does not change very rapidly, or at least it is usually not of interest to the well operator to detect such rapid changes if they occur. Accordingly, light pulses need to be sent from the optical source/detector 14 only occasionally, for example, every second, which provides an update of the temperature at the location of FBG 10 every second.
However, some parameters of interest to detect occur on much smaller time scale. For example, if the FBG 10 is used to measure a dynamic event, such as a pressure wave indicative of seismic activity occurring within the oil/gas well, sampling needs to take place more frequently. For example, a seismic pressure wave may contain frequency components as high as f=1000 Hz, and therefore would require interrogating the FBG 10 one the order of at least 2f times a second to properly resolve these higher order frequency components and to provide an accurate picture of the detected pressure wave. However, such high frequency rate pulsed sampling may not be possible in a practical application. For example, the FBG 10 will likely in an oil/gas application be wavelength-division or time-division multiplexed to other optical sensors such as flow rate meters, speed of sound meters, or other pressure or temperature sensors, and such meters or sensors may themselves contain FBGs which will produce reflections. (Examples of such other meters or sensors, and ways of multiplexing and interrogating them, are disclosed in the following U.S. patents or patent application, which are hereby incorporated by reference in their entireties: Ser. No. 09/740,760, filed Nov. 29, 2000; Ser. No. 09/726,059, filed Nov. 29, 2000; Ser. No. 10/115,727, filed Apr. 3, 2002; U.S. Pat. No. 6,354,147, issued Mar. 12, 2002). High rate sampling of FBG 10 could interfere with interrogation of the other optical sensors or meters multiplexed with FBG 10, and/or confused the reflected signals, making it difficult to determine which reflections pertain to which meter or sensor.
As alluded to above, one solution to the problem of interrogating the FBG 10 to monitor dynamic events is to interrogate the FBG 10 with a continuous wave light source. Continuous wave interrogation produces a continuous reflection of Bragg wavelengths shifts from the FBG 10, which can be monitored as a function of time. However, continually monitoring Bragg wavelength shifts is difficult in many applications, and requires detectors and signal processing schemes that are not always economical in practice.
Accordingly, there is room for improvement in the art of optical sensors. The art would benefit from a sensor design which can monitor dynamic events in real time, and which is interrogatable using methods that are easily implemented and reliable.